My Sierra ABS ECU has just started to fail the self-test and I cannot work out why. It all appears to work correctly up until that point.

I have gone through all tests listed in this post
https://passionford.com/forum/technical-help-q-and-a/374177-abs-light-2wd-cossie.html
especially those detailed by jon@work and all pass but my two dashboard ABS lights remain on - my red handbrake light does not come on at all.

The self-test appears to test:

• Low fluid in the reservoir – mine checks out OK and circuit completes
• Low pressure in the pump accumulator – Mine pumps up and the pump turns off correctly completing the circuit
• Valve block resistance – mine is within ohms range
• The four wheel ABS sensors – mine check out OK within ohms range

Does anyone know what else the self-test checks or can suggest any more checks as it seems to be that the ECU is not switching on relay 3 (green one) although that relay all checks out (when jumped)?

FYI. The Sierra diagnostic socket cannot interrogate ECU error codes as none of the wires go to the ECU. What they do test, all checks out anyway. I have also tried a proper (Crypton) ABS diagnosis loaned by a friendly garage and it fails to register any codes.

Any clue would be appreciated. Yes, the ECU itself could have failed but how does one test that out?